Up until last week, my program had distinct "this is speedwork" phases and "this is long run" phases. Speedwork was fairly specific. long runs were too, in the "Run 2.5 hours or 25% of your weekly mileage" type specificity.
Lines are getting a little blurry now... yesterday I ran about 16 miles (15.8, I think) with a warm up, four intervals of five minutes at about my half marathon pace with a one minute rest, an hour of easy running, and then 15 minutes back at half marathon pace.
It's fun, though exhausting. Typically, my long runs were long and my hard runs were hard, and I didn't do all that much in my program to merge them, working on different energy systems and so forth. But I think this is working on more- there's a mental discipline aspect, too.
Still, my body's going through a bit of a wringer. Because of certain scheduling issues, it's been nine days and 95 (or so) miles since my last rest day, and I'm thinking I won't get a day off till Sunday.
But that's part of the joy of running... figuring out how to get the miles in!